This invention relates to compositions for use as thermoelectric cooling and power generation elements and to methods for their preparation.
The field of thermoelectric cooling is an important one, having obvious commercial applications, such as in refrigerators. Semiconductor devices, which utilise the Peltier effect, are known, the primary example being Bi Te based devices Such devices, which are in widespread commercial use, employ suitably doped Bi Te alloys as both p and n type semiconductor elements. Sb and/or Se may be used as additives in order to modify the physical (primarily thermal) characteristics of the elements.
The efficiency of such thermoelectric cooling devices (i.e. the heat absorbed per unit of consumed power) is generally represented by a figure of merit or performance factor Z, which is given by:   Z  =                    S        2            ⁢      σ        κ  
where S is the thermopower coefficient or Seebeck voltage (V/xc2x0C.), "sgr" is the electrical conductivity (xcexa9xe2x88x921 cmxe2x88x921) and xcexa is the thermal conductivity (W cmxe2x88x922 or xc2x0C. cmxe2x88x921).
It is generally accepted that with the Bi Te based systems further improvements in Z are unlikely. Thus, there is a need to provide new materials having improved values of Z. U.S. Pat. No. 5,275,001 describes alternative materials for thermoelectric cooling which are based around various complex Sr Ti oxides. These materials are n type semiconductors.
The present invention addresses the abovementioned need by providing thermoelectric cooling elements having p type semiconductors exhibiting high values of Z. Furthermore, the present invention provides methods and strategies which enable the figure of merit Z to be optimized, by optimizing the combination of thermopower and electrical conductivity.
Goncalves et al (A P Goncalves, I C Santos, E B Lopes, R T Henriques, M Almeida and M O Figueiredo, Phys. Rev. B 37 (1988) 7476) disclose compositions of the general formula Y1xe2x88x92xPrxBa2Cu3O7xe2x88x92xcex4, where 0xe2x89xa6xxe2x89xa61. Measurements of thermopower and resistivity are made, for the purpose of explaining the presencexe2x80x94or absencexe2x80x94of superconductivity in these materials. Furthermore, the precise oxygen deficiencies xcex4 of the reported compositions are not established. Further still, the distribution of Y and Pr with the composition is homogeneous.
Macklin and Moseley (W J Macklin and P T Moseley, Materials Science and Engineering B7 (1990) 111) described the thermoelectric data available at the time concerning a number of complex copper oxides, and commented generally upon the prospects of using such oxides in thermoelectric applications.
It is well known that it is possible to generate electrical power by xe2x80x9creversingxe2x80x9d the above described process, i.e., by applying heat to materials of the type described above it is possible to generate electrical power.
According to a first aspect of the invention, there is provided a composition described by the formula
(RBa2Cu3O7xe2x88x92xcex4)x+(PrBa2Cu3O7xe2x88x92xcex4)1xe2x88x92x
wherein
R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or Lu;
0 less than xxe2x89xa61; and
xcex4 is such that the RBa2Cu3O7xe2x88x92xcex4 component of the composition is in a metallic phase.
Compositions as defined above may be used as thermoelectric cooling elements. These mixed oxide compositions exhibit large values of Z, since the presence of the Pr oxide (which is a semiconductor irrespective of the oxygen deficiency) results in a high thermopower value, whilst the presence of the Y oxide in a metallic phase results in high values of electrical conductivity.
Preferably, x is less than 0.4, most preferably x is in the range 0.10xe2x89xa6xxe2x89xa60.25.
The invention also provides thermoelectric cooling devices in which at least one cooling element comprises a composition as defined above, and thermoelectric power generation devices in which at least one power generation element comprises a composition as defined above.
According to a second aspect of the invention, there is provided the use of a composition described by the formula
RBa2Cu3O7xe2x88x92xcex4
wherein:
R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or Lu
xcex4 is such that the composition is in a transitional state between semiconducting and metallic phases; as a thermolectric cooling element or a thermoelectric power generation element.
Oxygen deficiencies corresponding to the phase transition from metallic to semiconductor behaviour (with a corresponding transition from orthorhombic to tetragonal lattice structures)xe2x80x94result in an optimal combination of electrical conductivity and thermopower.
The composition may be YBa2Cu3O7xe2x88x92xcex4, Eu Ba2Cu3O7xe2x88x92xcex4 or SmBa2Cu3O7xe2x88x92xcex4.
The invention also provides a thermoelectric cooling device in which at least one cooling element comprises a composition as defined with regard to this second aspect of the invention, and a thermoelectric power generation device in which at least one power generation element comprises a composition as defined with regard to this second aspect of the invention.
According to a third aspect of the invention, there is provided a method for preparing compositions as previously defined, the method comprising cooling of the composition by quenching.
The quenching may comprise quenching of the composition on a alumina plate, a copper plate or in liquid nitrogen.
According to a fourth aspect of the invention, there is provided a method for preparing compositions as previously defined, the method comprising the step of sintering granules of a predetermined range of grain sizes. The predetermined grain sizes may be in the range 0.1 to 100 xcexcm, preferably 0.1 to 30 xcexcm, most preferably 0.1 to 2 xcexcm. This fourth aspect of the invention may also include the third aspect of the invention.
According to a fifth aspect of the invention, there is provided a method for preparing compositions as previously defined in which ultrasonic treatment is employed.
According to a sixth aspect of the invention, there is provided a thermoelectric cooling device in which at least one cooling element comprises a thin film semiconductor material, or a thermoelectric power generation device in which at least one power generation element comprises a thin film semiconductor material.
The films are convenient to prepare and provide easily controllable and uniform ceramic structures. Furthermore, it is possible to provide films having highly anisotropic electrical properties and pre-defined grain size and structure, thereby permitting optimization of the figure of merit. Still further, the heat generated by thermocooling is efficiently dissipated from thin films because of the inherently high surface area to volume ratios.
Preferably, the thickness of the film is less than 5 xcexcm, most preferably less than 1 xcexcm.
The semiconductor material may comprise a composition as previously defined in respect of the other aspects of the invention.
The semiconductor material may be texturised.